FIELD OF THE INVENTION
The invention relates to a sheet conveyor belt and in particular to a device for conveying printing substrates, sheets of paper, plastic or other materials which are to be printed and/or have been printed, the device including a frame, a plurality of spaced-apart and axially parallel cylindrical rollers which are supported on the frame, one of the rollers being a rotatably drivable drive roller, and the other rollers being freely rotatably supported, and an endless conveyor belt that wraps around all the rollers and in operation is in static-frictional contact with the drive roller.
A device of the foregoing general type heretofore known, for example, from the published European Patent Document EP-A 0 738 680, has some advantages over conventional belt conveyor devices with grippers which make it especially well suited for use in conjunction with electrographic printing units, ink jet printing units, or other printing units which are controllable pixel by pixel.
Belt conveyor devices for printing presses have been developed departing from the basic form thereof wherein two rotatable rollers, one of which is driven, are disposed at a spaced distance from one another corresponding to the conveying distance. A conveyor belt is wrapped around both rollers with initial tension. While printing substrates are fed from a feeder to a delivery along the upper portion of the belt, for multicolor printing in register they pass through a plurality of printing units, at each of which the various basic colors are applied successively. Except for the fact that additional rollers are often provided on the inner side of the conveyor belt in order to support or apply tension to the belt, this basic form has remained unchanged to the present day.
Between the drive roller and the conveyor belt entrained therewith by friction, there is intrinsically a variable amount of slip, which cannot be reduced arbitrarily, especially because it is desirable for the inner side of the conveyor belt to be as smooth as possible so as to keep the friction low in the overall system. In particular, this slip cannot be ignored if high-precision registration is demanded. Heretofore known slip compensating devices are relatively complicated and thus can be considered only for very high-quality printing presses. A proposal is made in the aforementioned published European Patent Document EP-A 0 738 680, for example, that the lagging of the conveyor belt be detected by a sensor and that the printing signals to the printing units be delayed accordingly, so that the accuracy of registration is preserved.
In addition to the aforedescribed problems of slip between the drive roller and the conveyor belt, a problem exists in the heretofore known belt conveyor devices for printing presses that changing a worn or defective conveyor belt is a difficult task. Attempts to solve this problem so far by adhesively fastening or gluing the conveyor belt in the press, partly dismantling the press, or using split shafts, provide only inadequate solutions to the problem.